Java并发之线程的使用以及构建启动线程

构建线程

Thread说明

• 线程是程序中的执行线程，java虚拟机允许应用程序并发的运行多个线程。

• 每个线程都有一个优先级，高优先级线程的执行优先于低优先级线程。每个线程都可以或不可以标记为一个守护程序。当某个线程中运行的代码创建一个新 Thread 对象时，该新线程的初始优先级被设定为创建线程的优先级，并且当且仅当创建线程是守护线程时，新线程才是守护程序。

• 当 Java 虚拟机启动时，通常都会有单个非守护线程（它通常会调用某个指定类的 main 方法）。Java 虚拟机会继续执行线程，直到下列任一情况出现时为止：  
             1.调用了 Runtime 类的 exit 方法，并且安全管理器允许退出操作发生。  
             2.非守护线程的所有线程都已停止运行，无论是通过从对 run 方法的调用中返回，还是通过抛出一个传播到 run 方法之外的异常。

• 实现线程的方式，会在后续的章节中介绍

源码参考如下：

/**
 * A <i>thread</i> is a thread of execution in a program. The Java
 * Virtual Machine allows an application to have multiple threads of
 * execution running concurrently.
 * <p>
 * Every thread has a priority. Threads with higher priority are
 * executed in preference to threads with lower priority. Each thread
 * may or may not also be marked as a daemon. When code running in
 * some thread creates a new <code>Thread</code> object, the new
 * thread has its priority initially set equal to the priority of the
 * creating thread, and is a daemon thread if and only if the
 * creating thread is a daemon.
 * <p>
 * When a Java Virtual Machine starts up, there is usually a single
 * non-daemon thread (which typically calls the method named
 * <code>main</code> of some designated class). The Java Virtual
 * Machine continues to execute threads until either of the following
 * occurs:
 * <ul>
 * <li>The <code>exit</code> method of class <code>Runtime</code> has been
 *     called and the security manager has permitted the exit operation
 *     to take place.
 * <li>All threads that are not daemon threads have died, either by
 *     returning from the call to the <code>run</code> method or by
 *     throwing an exception that propagates beyond the <code>run</code>
 *     method.
 * </ul>
 * <p>
 * There are two ways to create a new thread of execution. One is to
 * declare a class to be a subclass of <code>Thread</code>. This
 * subclass should override the <code>run</code> method of class
 * <code>Thread</code>. An instance of the subclass can then be
 * allocated and started. For example, a thread that computes primes
 * larger than a stated value could be written as follows:
 * <hr><blockquote><pre class="brush:php;toolbar:false">
 *     class PrimeThread extends Thread {
 *         long minPrime;
 *         PrimeThread(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 *
 *         public void run() {
 *             // compute primes larger than minPrime
 *              . . .
 *         }
 *     }
 * 

---

*

* The following code would then create a thread and start it running: *

 *     PrimeThread p = new PrimeThread(143);
 *     p.start();
 * 

*

* The other way to create a thread is to declare a class that * implements the Runnable interface. That class then * implements the run method. An instance of the class can * then be allocated, passed as an argument when creating * Thread, and started. The same example in this other * style looks like the following: *

---

 *     class PrimeRun implements Runnable {
 *         long minPrime;
 *         PrimeRun(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 *
 *         public void run() {
 *             // compute primes larger than minPrime
 *              . . .
 *         }
 *     }
 * 

---

*

* The following code would then create a thread and start it running: *

 *     PrimeRun p = new PrimeRun(143);
 *     new Thread(p).start();
 * 

*

* Every thread has a name for identification purposes. More than * one thread may have the same name. If a name is not specified when * a thread is created, a new name is generated for it. *

* Unless otherwise noted, passing a {@code null} argument to a constructor * or method in this class will cause a {@link NullPointerException} to be * thrown. * * @author unascribed * @see Runnable * @see Runtime#exit(int) * @see #run() * @see #stop() * @since JDK1.0 */ publicclass Thread implements Runnable {

需要的信息

       在运行线程之前首先要构造一个线程对象，线程对象在构造的时候需要提供线程所需要的属性，如线程所属的线程组、线程优先级、是否是Daemon线程等信息。在new Thread时会调用以下方法进行实例化Thread对象。
初始化代码如下：

    /**
     * Initializes a Thread.
     *
     * @param g the Thread group
     * @param target the object whose run() method gets called
     * @param name the name of the new Thread
     * @param stackSize the desired stack size for the new thread, or
     *        zero to indicate that this parameter is to be ignored.
     * @param acc the AccessControlContext to inherit, or
     *            AccessController.getContext() if null
     */
    private void init(ThreadGroup g, Runnable target, String name,                      
    long stackSize, AccessControlContext acc) {        
    if (name == null) {            
    throw new NullPointerException("name cannot be null");
        }        
        this.name = name;        //当前线程作为该线程的父线程
        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();        //线程组的获取：如果传入的参数为空首先获取系统默认的安全组，如果为空获取父线程的安全组
        if (g == null) {            
        /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();        /*
         * Do we have the required permissions?
         */
        if (security != null) {            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();        
        this.group = g;        //设置daemon 、priority 属性为父线程对应的值
        this.daemon = parent.isDaemon();        
        this.priority = parent.getPriority();        
        
        if (security == null || isCCLOverridden(parent.getClass()))            
        this.contextClassLoader = parent.getContextClassLoader();        
        else
            this.contextClassLoader = parent.contextClassLoader;        
            this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();        
                this.target = target;
        setPriority(priority);        //将父线程的InheritableThreadLocal复制过来
        if (parent.inheritableThreadLocals != null)            
        this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);        
                /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;        /* Set thread ID */ 
        //生成线程id（一个long型的字段threadSeqNumber）
        tid = nextThreadID();
    }

结论

        一个新构建的Thread对象（new Thread()）,是由其父线程（当前线程）进行空间分配，而子线程继承了父线程的Daemon、优先级和加载资源的contextClassLoader，以及可继承的ThreadLocal，同时会为子线程分配一个线程id。一个可以运行的线程对象完成初始化工作，并且在堆内存中等待运行。

构建的方式

继承Thread

代码

//方法1通过继承Thread实现class MyThread extends Thread{

    //需要实现的方法，该方法执行具体的业务逻辑
    @Override    public void run() {
        System.out.println(Thread.currentThread().getName()
                +" @@@@ MyThread。run（）我是通过继承Thread实现的多线程");
    }

}

说明

      通过Thread源码发现（Thread implements Runnable）发现thread其实也是一个实现了runnable接口的一个实例，它代表一个线程的实例，并且，启动线程的唯一方法就是通过Thread类的start()实例方法。start()方法是一个native方法，它将启动一个新线程，并执行run()方法。这种方式实现多线程很简单，通过自己的类直接extend Thread，并复写run()方法，就可以启动新线程并执行自己定义的run()方法。

      其中run()方法的方法体代表了线程需要完成的任务，称之为线程执行体。当创建此线程类对象时一个新的线程得以创建，并进入到线程新建状态。通过调用线程对象引用的start()方法，使得该线程进入到就绪状态，此时此线程并不一定会马上得以执行，这取决于CPU调度时机。

实现接口Runnable

代码

//方法2通过实现runnable接口
//实现Runnable接口，并重写该接口的run()方法，该run()方法同样是线程执行体，创建Runnable实现类的实例，
//并以此实例作为Thread类的target来创建Thread对象，该Thread对象才是真正的线程对象。class MyRunnable implements Runnable{

    @Override    public void run() {
        System.out.println(Thread.currentThread().getName()+                
        " @@@@ MyRunnable。run（）我是通过实现Runnable接口实现的多线程");
    }

}

使用Callable、Future实现有返回结果的多线程

      使用Callable和Future接口创建线程。具体是创建Callable接口的实现类，并实现clall()方法。并使用FutureTask类来包装Callable实现类的对象，且以此FutureTask对象作为Thread对象的target来创建线程。
      可返回值的任务必须实现Callable接口，类似的，无返回值的任务必须Runnable接口。执行Callable任务后，可以获取一个Future的对象，在该对象上调用get就可以获取到Callable任务返回的Object了，再结合线程池接口ExecutorService就可以实现传说中有返回结果的多线程了（关于Executor的使用后续的文章中详细介绍。）。

//方法3通过Executor框架实现class MyCallable implements Callable<Integer>{
    //需要实现call方法而不是run方法
    @Override    public Integer call() throws Exception {        return 100;
    }
}

启动线程

通过源码分析得出：

• 1.对象初始化完成之后，通过执行start方法来执行这个线程，并且java虚拟机会调用该线程的run方法执行线程的业务逻辑；

• 2.调用start方法之后发现会同时有两个线程在执行：当前线程（parent线程【同步告知java虚拟机，只要线程规划器空闲，应立即启动调用start方法的线程】，从调用返回给start方法）和另一个线程（执行其run方法）。

• 3.并且多次启动一个线程是非法的。特别是当线程已经结束执行后，不能再重新启动。

  start方法源码说明如下：

   /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        /* Notify the group that this thread is about to be started
         * so that it can be added to the group&#39;s list of threads
         * and the group&#39;s unstarted count can be decremented. */
        group.add(this);        boolean started = false;        try {
            start0();
            started = true;
        } finally {            try {                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

    private native void start0();

参考代码

public class TestCreateThread {    public static void main(String[] args) {
        Thread myThread = new MyThread();
        myThread.setName("myThread");
        myThread.start();

        Runnable myRunnable = new MyRunnable();
        Thread myRunnableThread = new Thread(myRunnable);
        myRunnableThread.setName("myRunnableThread");
        myRunnableThread.start();

        Thread myRunnableThread2 = new MyThread(myRunnable);
        myRunnableThread2.setName("myRunnableThread2");
        myRunnableThread2.start();        //执行结果参考如下：
        //myThread @@@@ MyThread。run（）我是通过继承Thread实现的多线程
        //myRunnableThread2 @@@@ MyThread。run（）我是通过继承Thread实现的多线程
        //myRunnableThread @@@@ MyRunnable。run（）我是通过实现Runnable接口实现的多线程

        //测试callable方法
        // 创建MyCallable对象
        Callable<Integer> myCallable = new MyCallable();    
        //使用FutureTask来包装MyCallable对象
        FutureTask<Integer> ft = new FutureTask<Integer>(myCallable); 
        //FutureTask对象作为Thread对象的target创建新的线程
        Thread thread = new Thread(ft);
        thread.start();//启用

        //获取信息
        try {            //取得新创建的新线程中的call()方法返回的结果
            //当子线程此方法还未执行完毕，ft.get()方法会一直阻塞，
            //直到call()方法执行完毕才能取到返回值。
            int sum = ft.get();
            System.out.println("sum = " + sum);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }        //使用ExecutorService处理多线程
        ExecutorService pool = Executors.newFixedThreadPool(10);  
        Future<Integer> f = pool.submit(myCallable);  
        // 关闭线程池  
        pool.shutdown(); 
        try {            int sum1 = f.get();
            System.out.println("sum1 = " + sum1);
        } catch (InterruptedException e) {            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

启动线程的注意事项

      无论何种方式，启动一个线程，就要给它一个名字！这对排错诊断系统监控有帮助。否则诊断问题时，无法直观知道某个线程的用途。

Thread与Runnable的关系

实现关系

Thread实现接口Runnable，并且实现了run方法，代码参考如下:

        //如果该线程是使用独立的 Runnable 运行对象构造的，则调用该 Runnable 对象的 run 方法；
        //否则，该方法不执行任何操作并返回。
        //Thread 的子类应该重写该方法。
        /**
         * If this thread was constructed using a separate
         * <code>Runnable</code> run object, then that
         * <code>Runnable</code> object&#39;s <code>run</code> method is called;
         * otherwise, this method does nothing and returns.
         * <p>
         * Subclasses of <code>Thread</code> should override this method.
         *
         * @see     #start()
         * @see     #stop()
         * @see     #Thread(ThreadGroup, Runnable, String)
         */
        @Override
        public void run() {            if (target != null) {
                target.run();
            }
        }

}

区别

      当执行到Thread类中的run()方法时，会首先判断target是否存在，存在则执行target中的run()方法，也就是实现了Runnable接口并重写了run()方法的类中的run()方法。当时如果该Runnable的子类是通过一个继承Thread的子类（该且重写了run方法），则真正执行的是Thread子类重写的run方法（由于多态的原因）。

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